1. Field of the Invention
The present invention relates generally to a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and method for supporting slow forward power control on an SCH (Supplemental Channel) for transmitting a large amount of data in a BTS (Base station Transceiver System) and a BSC (Base Station Controller).
2. Description of the Related Art
A discontinuous transmission (DTX) mode refers to a mode in which data is transmitted in frames only when transmission data is generated in a wired system or a mobile communication system. Data transmission in the DTX mode minimizes transmission power and increases the whole system capacity due to the decrease of interference with the system.
The DTX mode, however, exhibits a problem when a receiver cannot determine whether frames have been transmitted or not due to the irregular transmission of frames from a transmitter. This makes it impossible for a BTS to perform a forward power control. More specifically, when a receiver in a mobile station (MS) cannot make a proper determination about data transmission, it cannot rely on decoder decision parameters including CRC (Cyclic Redundancy Code) and decoding results. Hence, there is no known method for controlling the transmission power of the MS accurately in a discontinuous Transmission (DTX) mode.
Both a DCCH (Dedicated Control Channel) and an SCH support the DTX mode. The DCCH is characterized by data transmission only when transmission data is generated in an upper layer, which makes the DCCH suitable as a control channel for efficient packet services. The DCCH is supposed to transmit null frames for power control during the DTX period. The SCH supports a DTX mode in which no data is transmitted in the absence of transmission data. The SCH transmits no frames during the DTX period.
FIG. 1 is a block diagram of a typical mobile communication system. The mobile communication system depicted in FIG. 1 is a reference model of 3G IOS (Interoperability Specifications) with an MSC (Mobile Switching Center), BSs (Base Stations), and a digital air interface between the BSs, which are well known.
Referring to FIG. 1, an interface A1 is defined for signaling and an interface A2/A5 (exclusively for circuit data) for user traffic between an MSC 20 and a BSC 32. An interface A3 is defined to connect a target BS 40 to an SDU (Frame Selection/Distribution Unit Function) 34 of a source BS 30 to implement a soft/softer handoff.
Signaling messages and user data are transmitted between the target BS 40 and the SDU 34 of the source system 30 by the interface A3. An interface A7 is defined for signal transmission/reception between the target BS 40 and the source BS 30 for inter-BS soft/softer handoff. The wired communication lines of this CDMA mobile communication system include a forward link directed from the MSC 20 to the BS 30, a reverse link directed from the BS 30 to the MSC 20, and a line between the BSs 30 and 40. The MSC 20 includes a call control and mobility management block 22 and a switching block 24. The MSC 20 is connected to a data network (not shown) such as the Internet through an IWF (InterWorking Function) 50. Interfaces A8 and A9 are defined for user traffic and signaling, respectively, between a BS and a PCF (Packet Control Function) 60, and interfaces A10 and A11, for user traffic and signaling, respectively, between the PCF 60 and a PDSN (Packet Data Serving Node) 70.
FIG. 2 is a diagram showing an SCH signal flow between a BTS and a BSC (BSC-SDU) in a conventional technology. This operation may occur between a BSC 32 (BSC-SDU 34) and a BTS 36 in the source BS 30 or a BSC 42 and a BTS 44 in the target BS 40.
Referring to FIG. 2, the BTS determines the type of a frame to transmit to the BSC and generates a reverse SCH message in step 11. The reverse SCH message is supposed to be transmitted to the BSC in every predetermined period (e.g. 20 ms) in response to a reverse SCH frame received in the predetermined period from an MS (not shown). Step 11 will be described later in more detail referring to FIGS. 3A and 3B. In step 12, the BTS transmits the reverse SCH message to the BSC. The reverse SCH message may contain a data/null/idle/erasure frame. The BSC receives and processes the reverse SCH message and generates a forward SCH message in step 13. Reception of the reverse SCH message will be described later in more detail referring to FIG. 5. Processing the reverse SCH message and generation of the forward SCH message will be described in further detail referring to FIGS. 4A and 4B. In step 14, the BSC transmits the forward SCH message to the BTS. The forward SCH message may contain a data/null/idle frame. The BTS performs a forward/reverse power control for the MS based on power control information included in the forward SCH message in step 15. Reception of the forward SCH message will later be described in more detail referring to FIG. 6.
To summarize the operation shown in FIG. 2, after receiving a data frame in every predetermined period (20 ms) from the MS, the BTS generates a reverse SCH message in the predetermined period and transmits it to the BSC. The BSC processes the reverse SCH message, generates a forward SCH message, and transmits it to the BTS. Then, the BTS performs a power control for the MS based on power control information included in the forward SCH message.
FIG. 3 is a flowchart illustrating a conventional reverse SCH message transmitting operation. In this operation, the BTS transmits a frame received in the predetermined period from the MS as a reverse SCH message to the BSC-SDU. The following description is conducted with the appreciation that a forward/reverse SCH message is constructed in the same format as an FCH (Fundamental channel) shown in FIGS. 7 to 10.
Referring to FIG. 3, the BTS determines whether it has secured radio resources related with the MS and acquired the MS in step 101. If it has not, the BTS considers that it is not being synchronized with the MS and sets Frame Content in an IS-2000 reverse SCH message shown in FIG. 10 to an idle frame to synchronize with the BSC-SDU in step 104. Since the BTS is being synchronized with the BSC-SDU, the BTS sets power control information (Frame Quality Indicator, FQI and Reverse Link Quality) to negligible values in the reverse SCH message that will be transmitted to the BSC-SDU in step 105. For example, FQI in the reverse SCH message is set to 0 and Reverse Link Quality to 0000000. In step 106, the BTS transmits the IS-2000 reverse SCH message to the BSC-SDU.
On the other hand, if the BTS has secured the radio resources related with the MS and acquired the MS in step 101, it checks the quality of a frame received from the MS in step 102. If the data frame is bad, the BTS sets Frame Content of the reverse SCH message to an erasure frame in step 104-1. In step 105-1, the BTS sets the power control information of the reverse SCH message to negligible values. For example, FQI in the reverse SCH message is set to 0 and Reverse Link Quality to 0000000. In step 106-1 the BTS transmits the IS-2000 reverse SCH message without any data to the BSC-SDU since the received frame is bad. Upon recognition of the erasure frame, the BSC-SDU requests the MS to increase its transmission power regarding reverse power control. That is, since the data frame received from the MS is bad, the BSC-SDU will request the MS to transmit a data frame with incremented power.
If the BTS determines that the received frame is good in step 102, it determines whether it detects a DTX mode during reception of a reverse SCH frame from the MS by a known DTX mode detection method applied to a radio transmission period between an MS and a BTS in step 103. If the DTX mode is detected, the BTS goes to step 104-3, and otherwise, it goes to step 104-2.
In step 104-2, the BTS sets Frame Content of the reverse SCH message to a data frame and in step 106-2, it sets power control information of the reverse SCH message based on the SCH frame received from the MS. In the case of forward power control, the BTS extracts PCBs (Power Control Bits) from a reverse pilot channel and performs an inner fast power control only when FPC_MODE=001 or 010 in the reverse SCH message shown in FIG. 10. In step 106-2, the BTS transmits the IS-2000 reverse SCH message with the data of the 20-ms data frame received from the MS encapsulated to the BSC-SDU.
Upon detection of a DTX mode in step 103, the BTS sets Frame Content of the reverse SCH message to a null frame in step 104-3. In step 105-3, the BTS sets FQI to 0 and Reverse Link Quality to the reception strength (Ec/Io) of the reverse pilot channel in the reverse SCH message. That is, if the reverse SCH is in the DTX mode, a reverse link power control is performed on the SCH based on the reverse pilot channel. On the other hand, in the case of forward power control, the BTS extracts PCBs from the reverse pilot channel and performs an inner fast power control only when FPC_MODE=001 or 010 in the reverse SCH message shown in FIG. 10. In step 106-3, the BTS transmits the IS-2000 reverse SCH message without any data to the BSC-SDU since the 20-ms frame received from the MS has no data.
FIGS. 4A and 4B are flowcharts illustrating a conventional forward SCH message transmitting operation. In this operation, the BSC-SDU transmits a forward SCH message to the BTS in every predetermined period (20 ms). It is to be noted in the following description that a forward/reverse SCH message is constructed in the same format as an FCH shown in FIGS. 7 to 10.
Referring to FIG. 4A, the BSC-SDU determines whether it has secured forward radio resources related with the MS and acquired the MS in step 201. If it has not, the BSC-SDU considers that it tries to synchronize with the MS and sets Frame Content in an IS-2000 forward SCH message of FIG. 8 to an idle frame to synchronize with the BTS in step 203. Since the BSC-SDU is being synchronized with the BTS, it sets power control information in the forward SCH message that will be transmitted to the BTS to appropriate values in step 206. Here, forward power control information (FPC: gain ratio) in the forward SCH message is set to an initial value for control of the MS and reverse power control information is internally set to a value identical to or proportional to Reverse: outer loop threshold (OLT) of a forward FCH/DCCH referring to power control information (FQI and Reverse Link Quality) included in a reverse SCH message received every 20 ms from the BTS. When necessary, reverse power control is performed based on Reverse: OLT of the forward FCH/DCCH. In step 207, the BSC-SDU transmits the forward SCH message with the set power control information to the BTS. Here, no data is loaded in the forward SCH message.
On the other hand, if the BSC-SDU has secured the radio resources related with the MS and acquired the MS in step 201, it checks whether there is data to be transmitted to the MS in the BSC or an external network element (e.g., PDSN(Packet Data Serving Node) or whether a DTX mode should be set on the forward link due to a bad SNR (signal-to-noise ratio) of a reverse pilot in step 202. If there is no data to transmit to the MS, the BSC-SDU goes to step 203-1 and if there exists data to transmit to the MS, it goes to step 203-2.
In step 203-1, the BSC-SDU sets Frame Content of the forward SCH message to a null frame. The BSC-SDU checks whether Frame Content of the latest reverse SCH frame received from the BTS indicates one of a null frame and an idle frame in step 204A. If it is neither a null frame nor an idle frame, the BSC-SDU checks whether Frame Content of the latest reverse SCH message indicates an erasure frame in step 205A. If it does not indicate an erasure frame, the BSC-SDU internally sets reverse power control information (Reverse: OLT) based on power control information (FQI and Reverse Link Quality) included in the reverse SCH message received from the BTS every 20 ms in step 206-1A. Since there is no forward control information in the reverse SCH message, a forward power control parameter, FPC: GR (Gain Ratio) is set based on FPC:SNR (Signal to Noise Ratio) included in a reverse FCH/SCH message. Since there is no data to transmit to the MS, the BSC-SDU loads no data in the forward SCH message and transmits it to the BTS in step 207-1.
If Frame Content of the latest reverse SCH message indicates an erasure frame in step 205A, the BSC-SDU sets a reverse power control information value to indicate power-up on a reverse link in the forward SCH message in step 206-2A. Since there exists no data to transmit to the MS, the BSC-SDU transmits the forward SCH frame without any data to the BTS in step 207-1.
If Frame Content of the latest reverse SCH message indicates one of a null frame and an idle frame in step 204A, the BSC-SDU maintains the power control information included in the reverse SCH message received from the BTS every 20 ms. The power control information is maintained until an erasure frame or a data frame is received from the BTS. That is, the BSC-SDU internally sets reverse power control information value to the previous value or to a value proportional to Reverse: OLT of the forward FCH/DCCH and a forward power control parameter, FPC: GR based on FPC: SNR of the reverse FCH/DCH in step 206-3A. Since there exists no data to transmit to the MS, the BSC-SDU transmits the forward SCH frame without any data to the BTS in step 207-1.
If there exists data to transmit to the MS in step 202, the BSC-SDU sets Frame Content of the forward SCH to a data frame in step 203-2 of FIG. 4B. Then, steps 204B to 206-3B are performed in the same manner as steps 204A to 206-3A.
In step 204B, the BSC-SDU checks whether Frame Content of the latest reverse SCH message is one of a null frame and an idle frame. If it is neither a null frame nor an idle frame, the BSC-SDU checks whether Frame Content of the latest reverse SCH message indicates an erasure frame in step 205B. If it does not indicate an erasure frame either, it sets the power control information in the forward SCH message based on power control information included in the reverse SCH message received from the BTS in step 206-1B. Since there is data to transmit to the MS, the BSC-SDU transmits the forward SCH message with the data to the BTS in step 207-2.
If the Frame Content of the latest reverse SCH message indicates an erasure frame in step 205B, the BSC-SDU sets the reverse power control information value to indicate power-up on the reverse link in the forward SCH message in step 206-2B. Since there is data to transmit to the MS, the BSC-SDU transmits the forward SCH frame with the data to the BTS in step 207-2.
If Frame Content of the latest reverse SCH message indicates one of a null frame and an idle frame in step 204B, the BSC-SDU maintains the power control information included in the reverse SCH message received from the BTS every 20 ms. The power control information is maintained until an erasure frame or a data frame is received from the BTS. That is, the BSC-SDU sets the power control information of the forward SCH message to the previous values in step 206-3B. Since there is data to transmit to the MS, the BSC-SDU transmits the forward SCH frame with the data to the BTS in step 207-2.
FIG. 5 is a flowchart illustrating a conventional reverse SCH message receiving operation. In this operation, the BSC-SDU receives and processes a reverse SCH message in every predetermined period (e.g., 20 ms) from the BTS.
Referring to FIG. 5, the BSC-SDU receives a reverse SCH message from the BTS every 20 ms in step 300. The BSC-SDU determines whether Frame Content of the received message indicates an erasure frame in step 301. If the received frame is an erasure frame, the BSC-SDU goes to step 304 and otherwise, it goes to step 302. In the case of an erasure frame, this implies that a frame received at the BTS from the MS is bad. Therefore, the BSC-SDU neglects all information in the received reverse SCH message and generates a forward SCH message indicating reverse power-up in step 304.
If the received reverse SCH frame is not an erasure frame in step 301, the BSC-SDU determines whether Frame Content of the received frame indicates an idle frame in step 302. In the case of an idle frame, the BSC-SDU neglects all information of the received reverse SCH message and generates a forward SCH message with reverse power control information maintained at an initial value, considering that the BTS has not recognized the radio resources related with the MS or has not assigned the radio resources, and with FPC: GR based on the energy of the reverse pilot channel in step 304-1.
If the received reverse SCH message is not an idle frame in step 302, the BSC-SDU determines whether its Frame Content indicates a null frame in step 303. In the case of a null frame, the BSC-SDU neglects all information of the received reverse SCH message and generates a forward SCH message with reverse power control information maintained at a value set just before a DTX mode is recognized, considering that a reverse channel between the MS and the BTS is in the DTX mode in step 304-2. The BSC-SDU also sets FPC: GR based on the energy of the reverse pilot channel because the reverse SCH message has no forward power control information. That is, the BSC-SDU neglects the power control information of the reverse SCH message and sets power control information before the DTX detection as reverse power control information for the MS in step 304-2.
If the reverse SCH message is not a null frame in step 303, which implies that it is a data frame, the BSC-SDU transmits data included in Reverse Link Information of the reverse SCH message to a corresponding data processing device (not shown) according to the type of the data and adjusts Reverse: OLT of the forward FCH/DCCH message referring to the reverse power control information. The BSC-SDU also sets FPC: GR based on the energy of the reverse pilot channel because the reverse SCH message has no forward power control information. That is, the BSC-SDU determines reverse power control information for the MS by analyzing the power control information of the reverse SCH message in step 304-3.
FIG. 6 is a flowchart illustrating a conventional forward SCH message receiving operation. In this operation, the BTS receives and processes a forward SCH message in every predetermined period (e.g., 20 ms) from the BSC-SDU.
Referring to FIG. 6, the BTS receives a forward SCH message from the BSC every 20 ms in step 400. The BTS determines whether Frame Content of the received message indicates an idle frame in step 401. In the case of an idle frame, the BTS analyses all information of the received forward SCH message, uses the reverse power control information of the forward FCH/DCCH message as reverse power control information, and uses the forward power control information of the forward SCH message as forward power control information in step 403.
If the forward SCH message is not an idle frame in step 401, the BTS determines whether Frame Content of the forward SCH message indicates a null frame in step 402. In the case of a null frame, the BTS analyses all information of the received forward SCH message and transmits the reverse power control information of the forward FCH/DCCH message as reverse power control information and the forward power control information of the forward SCH message as forward power control information to a power control processor (not shown) in step 403-1. Here, the forward power control information is maintained at the value set before the null frame is received or adjusted to a value identical to or proportional to FPC: GR of the FCH/DCCH.
If the forward SCH message is not a null frame in step 402, which implies that it is a data frame, the BTS analyses all information of the received forward SCH message and transmits the reverse power control information of the forward FCH/DCCH message as reverse power control information and the forward power control information of the forward SCH message as forward power control information to the power control processor in step 403-2.
FIG. 7 illustrates the structure of a message transmitted from the BSC to the BTS on a user traffic sub-channel of an FCH. The message is used to transmit a forward traffic channel frame directed to the MS. This message can be transmitted between a BTS and a BSC in the same BS or between a BTS and a BSC in different BSs although the message is differently called according to the interfaces. For example, the message is called “Abis SCH Forward” in the former case and “A3 SCH Forward” in the latter case. The symbols shown therein are well known to those skilled in the art.
FIG. 8 illustrates an example information element, Forward Layer 3 SCH Data representing control information for a forward CDMA traffic channel frame and a packet directed from an SDU to a target BTS. The symbols shown therein are well known to those skilled in the art.
FIG. 9 illustrates a message transmitted from the BTS to the BSC on a user traffic sub-channel of an FCH. This message is used for the BTS to transmit a decoded reverse traffic channel frame and control information. The message can be transmitted between a BTS and a BSC in the same BS or between a BTS and a BSC in different BSs although the message is differently called according to the interfaces. For example, the message is called “Abis SCH Reverse” in the former case and “A3 SCH Reverse” in the latter case. Again, the symbols shown therein are well known to those skilled in the art.
FIG. 10 illustrates an example Reverse Layer 3 SCH Data representing control information for a reverse CDMA traffic channel frame and a packet directed from a target BTS to an SDU. The symbols shown therein are well known to those skilled in the art.
The above-described conventional method produces the following disadvantages in a BS.
1. Dependence of power control on FCH/DCCH: Power control on an SCH is performed in proportion to power control on an FCH/DCCH or depends on the FCH/DCCH although the SCH is different from the FCH/DCCH. Since the SCH is dedicated to data traffic, its FER requirement is higher than that for the FCH/DCCH used for both signaling and user traffic. Therefore, power control on the SCH depending on the FCH/DCCH is inaccurate.
2. Impossibility of a BS checking the status of an MS during a DTX period of a forward SCH: When the SCH is set to a DTX mode; the BS cannot check the status of the MS for the DTX period. This results in inaccurate power control for the DTX period and afterwards.
3. SCH slow power control and DCCH slow/fast power control when a forward SCH is established: Slow power control is not supported on the forward SCH in the conventional technology. The SCH power control is proportional to the FCH/DCCH power control or depends on the FCH/DCCH power control. There exists a need for defining a forward power control mode for the forward SCH slow power control and a method of supporting DCCH slow/fast power control.